U.S. Pat. No. 4,321,311 to Strangman describes a composite coating system for thermal barrier ceramic coatings on metal parts. The system has three interrelated elements. (1) a metallic layer of an alumina forming metal, such as MCrAlY alloy where M is Ni, Co, NiCo, Fe or other metals, (2) a continuous adherent alumina layer formed on the metallic layer, and (3) a discontinuous ceramic layer having a particular columnar morphology on the alumina layer.
The ceramic layer has columnar grains, oriented substantially perpendicular to the surface of the substrate and having free surfaces extending down to the alumina layer. The voids between the individual ceramic columns enlarge and diminish to accommodate strains arising because of the differential thermal expansion between the metallic and ceramic materials.
This patent teaches deposition cf the columnar ceramic coating by positioning the substrate over an electron-beam heated pool of molten ceramic in vacuum. The substrate temperature was maintained in the range 1000.degree. to 1500.degree. F. to provide a relatively coarse columnar structure. In order to assure that the ceramic coating contained the stoichiometric amount of oxygen, the dark, as-deposited ceramic was heat treated at about 1600.degree. F. for 32 hours in an oxygen containing atmosphere so that the color lightened to a shade typical of the stoichiometric ceramic.
A closely related U.S. Pat. No. 4,321,310 to Ulion and Ruckle, teaches polishing of the interface between the metallic layer and the alumina layer to provide a surface roughness of less than about 25 microinches RMS average. This increases the adherence of the columnar ceramic layer and, thus, increases the cyclic temperature durability of the coating system.
The prior art also teaches the deposition of ceramic materials, such as yttria, by electron-beam heated evaporation of a metal or alloy vapor in the presence of oxygen. See, for example, R. F. Bunshah, "Structure/property relationships in evaporated thick films and bulk coatings", J. Vac. Sci. Technol. Vol. 11, p. 633-39, July/Aug 1974.